The intensive introduction of advanced composite materials into primary structures for aircraft has become one of the priority objectives in the design and manufacturing of a new generation of aircraft due to the possibilities they provide for structural optimisation. As such, a large proportion of current aeronautical structures are made from composite materials.
As is well known, these composite materials have very low or zero electrical conductivity, with these electrical properties being insufficient to protect the different, generally metallic, components or equipment installed on the aforementioned aeronautical structures made from composite material, in the case that any electrical discharge takes place to them. In these cases, the composite material structures themselves are seriously damaged.
In the traditional cases in which the aeronautical structures are made from metallic material (generally aluminium alloys) the, typically metallic conducting components or equipment which need to be joined to these structures are connected directly by riveting or by means of conducting metallic joints. This solution, however, is not valid in the case that the aeronautical structure is made from composite material, given that it would not inherently provide a low impedance path for the electrical current from, for example, an electrical discharge to the structure, which would be capable of protecting both the metallic component or equipment and the composite material structure.
In the particular cases in which the composite material structures belong to parts of the aircraft at high risk from discharges, such as is the case for structures through which fuel for the aircraft passes, the direct installation of metallic components or equipment onto the composite material structure would provide a path for the electrical current with high electrical impedance, which would translate into irreversible damage in the component or equipment, as well as to the composite material structure itself, in the case of a strike by or the passage of current (for example, from a lightning strike to the aircraft structure). In addition, strikes from, for example, lightning, provide extremely high currents of up to 100,000 amperes.
The known solutions raise the problem that they either protect the metallic equipment or component or the composite material structure itself, but not both and not properly. In these known solutions, it is common to protect the composite material by means of a metallic mesh or layer, with the metallic equipment or component inherently having a degree of surface conductivity which permits the passage of current. However, these solutions are not appropriate, in particular in the interface or junction between the metallic equipment or component and the composite material structure: the problem lies in the fact that it is very complicated to arrange the aforementioned interface between the equipment and the structure so that the transfer of current takes place safely between the two elements.
On the other hand, these known solutions do not consider the case in which an electrical discharge occurs from electrical charge accumulated by an electrostatic effect: the surface protection of the metallic equipment or component is not taken into account, nor is it provided with sufficient residual conductivity capable of draining the accumulated electrostatic charge in a controlled way.
It would, therefore, be desirable to develop a configuration for the fastening of metallic components or equipment installed onto composite material structures with low conductivity, so as to provide an appropriate path so that the current either from a discharge or from the accumulated charge from an electrostatic effect can move between the structure and the component or equipment without causing damage to either of the two elements.
This invention is aimed at achieving the above objectives.